Hydrocarbon oil compositions typically comprise a mixture of at least one hydrocarbon base oil and one or more additives, e.g., dispersant additive, where each additive is employed for the purpose of improving the performance and properties of the base oil in its intended application; e.g., as a lubricating oil, heating oil, diesel oil, middle distillate fuel oil, and so forth.
Dispersants are typically polymeric materials with an oleophilic component providing oil solubility and a polar component providing dispersancy. Dispersants generally have a number average molecular weight ( M.sub.n) of 10,000 or less.
Dispersants used in lubricating oils typically are hydrocarbon polymers modified to contain nitrogen- and ester-based groups. Polyisobutylene is commonly used in the preparation of dispersants, although other hydrocarbon polymers, such as ethylene-.alpha.-olefin copolymers, can be employed as well. Dispersants are primarily used to maintain in a suspension in the oil any insolubles formed by oxidation, etc. during use, thereby preventing sludge flocculation and precipitation. The amount of dispersant employed is dictated and controlled by the effectiveness of the particular material in achieving its dispersant function.
Nitrogen- and ester-based dispersants can be prepared by first functionalizing a long-chain hydrocarbon polymer, e.g., polyisobutylene, with maleic anhydride to form the corresponding polymer substituted with succinic anhydride groups, and then derivatizing the succinic anhydride-substituted polymer with an amine or an alcohol or the like. Polyisobutylene generally contains residual unsaturation in amounts of about one ethylenic double bond per polymer chain, positioned along the chain. The ethylenic double bonds serve as sites for functionalizing the polyisobutylenes by, for example, the thermal "ene" reaction (i.e., by direct reaction with maleic anhydride or one or more other dicarboxylic acid moieties).
The polyisobutylene (PIB) polymers employed in conventional dispersants typically have a M.sub.n of from 900 to 2500. PIB having a M.sub.n of less than 300 gives rather poor performance results when employed in dispersants because the molecular weight is insufficient to keep the dispersant molecule fully solubilized in lubricating oils. On the other hand, high molecular weight PIB ( M.sub.n &gt;3000) can be too viscous and difficult to process in many operations. This problem becomes much more severe as the PIB molecular weight increases to 5,000 or 10,000.
Increased amounts of terminal ethylenic unsaturation in polyisobutylene (so-called "reactive polyisobutylene") have been achieved by BF.sub.3 -catalyzed polymerization of isobutylene, such as disclosed in U.S. Pat. No. 4,152,499. Nonetheless, reactive polyisobutylenes can still contain substantial amounts of unsaturation elsewhere along the chain. Furthermore, it is difficult to produce reactive polyisobutylene polymers at molecular weights of greater than 2,000.
UK Patent 1329334 exemplifies the use of a conventional Ziegler-Natta catalyst for the preparation of ethylene-.alpha.-olefin copolymers of relatively low molecular weight. The patent discloses the production of ethylene polymer wax by polymerizing ethylene and optionally an .alpha.-olefin in the presence of hydrogen using a catalyst composed of a titanium or vanadium halogen compound supported on a carrier (a hydrocarbon-insoluble Mg compound) and an organo-aluminum compound. The molecular weight and density of the polymer wax are controlled by the amount of hydrogen and/or .alpha.-olefin used in the polymerization. The polymer wax is disclosed to have a M.sub.n in the range of 400 to 20,000. The wax may be oxidized without the formation of cross-linkages due to the small content of double bonds in the wax, and the oxidized wax may be modified by reaction with a maleic acid compound. The patent contains an example disclosing the production of an ethylene-1-butene polymer wax containing 28 ethyl groups per 1000 carbon atoms, which is equivalent to about 94 mole % ethylene assuming the ethyl groups in the polymer are due to units derived from 1-butene.
Ethylene-.alpha.-olefin copolymers of low molecular weight and containing residual double-bond unsaturation have been prepared using a catalyst comprising a metallocene and an alumoxane. For example, U.S. Pat. No. 4,668,834 teaches ethylene-.alpha.-olefin copolymers and terpolymers having a M.sub.n of between about 250 and about 20,000, a viscosity index of at least about 75, a vinylidene-type terminal unsaturation and molar ethylene content in the range of between about 20 and about 80. Propylene and 1-butene are specifically disclosed to be among the preferred .alpha.-olefins for polymerization with ethylene.
Similarly, U.S. Pat. No. 4,704,491 relates to liquid ethylene-.alpha.-olefin random copolymers which can be produced by copolymerizing ethylene and a C.sub.3 -C.sub.20 .alpha.-olefin in the presence of a catalyst comprising a group IVb transition metal compound, such as a metallocene, and an aluminoxane. In addition to numerous examples directed to EP copolymers, the patent provides two examples of the preparation of EB copolymers by the polymerization of ethylene and 1-butene in the presence of zirconocene-aluminoxane catalyst systems. Example 6 discloses an EB copolymer having an ethylene content of 55 mole % (=38 wt. %) and an M.sub.n of 1200. Example 14 discloses an EB copolymer with 60 mole % ethylene (43 wt. %) and M.sub.n of 2300.
U.S. Pat. No. 5,043,515 teaches a zirconocene/aluminoxane catalyst for oligomerizing olefins and the oligomerization process using the catalyst. More particularly, the patent discloses the oligomerization of ethylene or ethylene with one or more C.sub.3 -C.sub.10 .alpha.-olefins using the catalyst. It is further disclosed that, when the starting material is ethylene in combination with one or more .alpha.-olefins, the product olefins (i.e., the oligomers) contain significant portions of vinylidene olefins. Example 3-5 of the patent describes the oligomerization of ethylene and 1 -butene using bis(cyclopentadienyl)zirconium dichloride and aluminoxane.
EP-A-353935, related to U.S. Pat. No. 5,229,022, is directed to oil-soluble lubricating oil additives comprising at least one terminally unsaturated ethylene-.alpha.-olefin polymer having a M.sub.n of 300 to 10,000 substituted with mono- or dicarboxylic acid producing moieties, wherein at least about 30% of the polymer chains of the ethylene-.alpha.-olefin polymer possess terminal ethenylidene unsaturation. EP-A-441548 provides similar teachings for terminally unsaturated ethylene-.alpha.-olefin copolymers having M.sub.n s from about 300 to 20,000. EP-A-353935 further discloses that the monocarboxylic acid and the dicarboxylic acid or anhydride substituted polymers can be further reacted with a nucleophilic reagent such as amines, alcohols, amino alcohols and metal compounds, to form derivatives useful as lubricating oil additives such as dispersants. Example 5 discloses the preparation of an EB copolymer with M.sub.n =860 using dimethylsilyldicyclopentadienyl zirconium dichloride and methylalumoxane. The ethylene/butene-1 copolymers are prepared so as to provide polymer compositions having a low amount of the unsaturation as trisubstituted unsaturation (sometimes referred to as trisubstituted vinyl).
U.S. Pat. No. 4,981,605 relates to liquid epoxidized ethylenic random copolymers and to liquid hydroxylated ethylenic random copolymers, both of which are useful as lubricant oil additives, paint additives, and resin modifiers. The patent discloses that the epoxidized/hydroxylated ethylenic random copolymer is an epoxidation/hydroxylation product of a liquid ethylenic random copolymer of ethylene and a C.sub.3 -C.sub.20 .alpha.-olefin, wherein the epoxy/hydroxyl groups are each formed via a carbon-carbon unsaturated bond derived from ethylene or the .alpha.-olefin and positioned at the polymer chain end of the liquid ethylenic random copolymer. The patent further discloses that the liquid ethylene random copolymer has, inter alia, an ethylene component content of 10-85 mole %, an .alpha.-olefin content of 15 to 90 mole %, a M.sub.n of usually 200 to 10,000, and a molecular weight distribution of usually not more than 4.0. Referential Example 6 discloses the preparation of a liquid EB random copolymer with an ethylene content of 58 mole % (41 wt. %) and M.sub.n of 1500 by polymerization of ethylene and 1-butene in the presence of bis(cyclopentadienyl) zirconium dichloride and aluminoxane.
Other publications which relate primarily to the chemical modification of metallocene-alumoxane-prepared, low molecular weight ethylene-.alpha.-olefin copolymers to provide additives for lubricating oils include, for example, U.S. Pat. No. 4,943,658 and U.S. Pat. No. 5,017,299.
U.S. Pat. No. 5,084,534 discloses high molecular weight polyethylenes incorporating a small portion 1-octene, 1-hexene, or 1-butene.
U.S. Pat. No. 4,849,572 discloses polybutene BF.sub.3 polymerization with immediate catalyst quench for high isobutylene conversion to PIB having at least 40% terminal unsaturation. Double bond types are discussed by varying hydrocarbon substitution.
It has been found that further improvements in the performance of ashless dispersants based on ethylene-.alpha.-olefin polymers, as well as significant improvements in the economics of the dispersants can be achieved by selectively controlling, for example, the specific co-monomer and its content and certain polymer properties, within the broad general class of ethylene-.alpha.-olefin copolymers.